Aqueous emulsion of fluorine-containing polymer

ABSTRACT

An aqueous emulsion of fluorine-containing polymer according to the present invention contains an emulsion stabilizer represented by the formula: 
     
         R--Ph--O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 --SO.sub.3 M 
    
     (Ph is a phenyl radical; R is an alkyl radical having 8-12 carbon atoms; n is a number of 1-6; and M is Na, K or NH 4 ). The resulting emulsion has excellent mechanical stabilities and thermal stabilities as well as decreased possibility of causing ground water contamination.

This application claims benefit of international application PCT/ JP95/01323 filed Jul. 3, 1995.

TECHNICAL FIELD

The present invention relates to a novel aqueous emulsion offluorine-containing polymer having excellent mechanical stability andthermal stability.

BACKGROUND ART

Aqueous emulsions of polytetrafluoroethylene (hereinafter referred to asPTFE) are prepared by the emulsion-polymerization method disclosed inthe U.S. Pat. No.2,559,752. Aqueous emulsions of otherfluorine-containing polymers are also prepared by using similar process.Aqueous emulsions of fluorine-containing polymer thus prepared havebeing employed as coating agents for metals or the like, impregnatingagents for fibers and cloths, additives for various materials, andfurther as dust-preventive treating agents. For adaptation to theseuses, the emulsion is generally stabilized with such nonionicsurfactants as p-C₈₋₁₀ alkylphenylpolyethyleneglycolethers, as theemulsion itself is inferior in mechanical stability. However, even thusstabilized emulsions become unstable against thermal and mechanicalactions they encounter during evaporation, concentration, dilution,transportation, weighing and others.

It is an object of the present invention to provide an aqueous emulsionof fluorine-containing polymer having excellent mechanical stability. Byuse of the inventive emulsion, for example, preventions become feasiblefor such problems caused by inferior mechanical stability as clogging ofpumps, valves and nozzles with coagulated colloidal particles offluorine-containing polymers formed during agitation, transportation andspraying, adhesion of coagulated particles to container walls andagitators, and flotation of coagulated particles in the emulsion.

It is another object of the present invention to provide an aqueousemulsion of fluorine-containing polymer having excellent thermalstability. By use of the inventive emulsion, for example, preventionsbecome feasible for such problems caused by inferior thermal stabilityas deteriorated coating property and impregnation property towardsmetal, glass woven fabric owing to increased emulsion viscosities underhigh temperatures.

It is a further object of the present invention to provide an aqueousemulsion of fluorine-containing polymer having reduced environmentalpollutions especially in the ground water pollution. By use of theinventive emulsion, for example, preventions become feasible for suchproblems as pollution of ground water caused by emulsion stabilizers inthe emulsion occurring when materials treated with PTFE emulsions areused for mixing with soil or filling in the ground for reclamation.Moreover, the inventive emulsion allows concentration by evaporation ordilution thereof, thanks to its increased mechanical and thermalstabilities.

DISCLOSURE OF THE INVENTION

An aqueous emulsion of fluorine-containing polymer according to thepresent invention is characterized by that it contains an emulsionstabilizer represented by the formula:

    R--Ph--O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 --SO.sub.3 M

in which Ph is a phenyl radical, R is a C₈ -C₁₂ alkyl radical, n is anumber of 1-6, and M is Na, K or NH₄.

The emulsion stabilizer is added preferably not less than 1 wt % to thepolymer.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the fluorine-containing polymer includespolymers of tetrafluoroethylene, chlorotrifluoroethylene orvinylidenefluoride, or copolymers containing the aforementioned. Mentionis made of the polymers including polytetrafluoroethylene,tetrafluoroethylene.hexafluoropropylene copolymer,tetrafluoroethylene.fluoroalkylvinylether copolymer,tetrafluoroethylene.ethylene copolymer, polychlorotrifluoroethylene,polyvinylidenefluoride and vinylidenefluoride.hexafluoropropylenecopolymer.

In the present invention, the aqueous emulsion means those containing inwater 1-75 wt % of the fluorine-containing polymer in colloidalparticles having an average particle size of 0.1-0.3μ. The averageparticle size of colloidal particles is measured by the centrifugalsedimentation method. In Examples and Comparative Examples of thepresent invention, the values were measured by use of a centrifugalsedimentation particle size distribution measuring equipment (SA-CP4L;Shimadzu Corp.).

As for the emulsion stabilizer, those having the formula

    R--Ph--O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 --SO.sub.3 M

are used. In the formula, Ph denotes a phenyl radical, R denotes analkyl radical having 8-12 carbon atoms being preferably an octyl radicalor a nonyl radical, and n for the oxyethylene radical denotes a numberof 1-6 being preferably 1-3. When n is larger than 6, excessivehydrophilic properties decrease the emulsifying efficiency to result inthe aqueous emulsion of fluorine-containing polymer having inferiormechanical stability and decreased thermal stabilities caused byenhanced effects of the oxyethylene radical. M is selected from Na, Kand NH₄ being preferably Na due to the resulting solubility to water.The amount of emulsion stabilizer contained is not less than 1.0 wt %per fluorine-containing polymer weight preferably being in a range of1.5-5 wt %. When the amount added is less than 1.0 wt %, the emulsiontends to become unstable. No upper limit exists for the amount to beadded, however, over 10 wt % is uneconomical and thus less than 5 wt %is preferred. The amount of emulsion stabilizer in the present emulsioncan be reduced to 1/2-1/3 of the amount of conventional emulsionstabilizer p-nonylphenylpolyethyleneglycolether.

The mechanical stability of the aqueous emulsion of fluorine-containingpolymer can be assessed by observation of increased ratio of apparentsizes of associated colloidal particles resulting from collisions ofparticles in the emulsion during high speed agitation of the emulsion.Further, under agitation, the colloidal particles coagulate partly toform a coagulum, which may adhere to agitator rotors, vessel walls andothers, or isolate from the emulsion as a flock. Under occurrence ofthese phenomenon, the emulsion is regarded as having an inferiorstability, despite of the increase or decrease in size of colloidalparticles remaining in the emulsion. In the present invention, themechanical stability is assessed by increased ratio of particle sizemeasured according to the following method. Into a beaker of 60 mm innerdiameter is charged 200 ml of an emulsion having an average particlesize (S₁). An agitator (ULTRA-TURRAX; JUNKE & KUNKEL GMBH & Co. KG)having a shaft with a generator is so disposed in the beaker as theshaft is apart by 15 mm from the bottom of beaker and deviates by 5 mmfrom the center, and the agitator is operated for 5 minutes under 20,500rpm. The average particle size (S₂) of emulsion after being subjected tothe agitation is measured, and the increased ratio of particle size iscalculated according to the following formula:

    Increased ratio of particle size (%)=(S.sub.2 -S.sub.1)/S.sub.1 ×100

The mechanical stability is rated under the following standard:

Good: Increased ratio of particle size is >0%-<5%;

Fair: Increased ratio of particle size is ≧5%-<10%; and

Poor: Increased ratio of particle size is ≧10%.

In addition to the above, Poor is rated when a large amount ofcoagulated matter is formed by the agitation.

Viscosity of the aqueous emulsion increases in accordance with increasein the temperature of emulsion. Unfavorable emulsions show sharpincrease in viscosity at temperatures the emulsion encounter when storedin summer, during usages or concentration by evaporation. In the Exampleand Comparative Example of the present invention, the viscosity wasmeasured at 20-60° C. for aqueous emulsions containing about 60 wt %fluorine-containing polymer by use of a B-Type viscometer (Type BL;Tokyo Keiki). Emulsions showing vigorous increases in viscosity at30-60° C. are recognized as having poor thermal stability.

EXAMPLES 1, 2 AND COMPARATIVE EXAMPLE 1

To aqueous emulsions containing about 45 wt % PTFE prepared by thecontrolled emulsion polymerization method (the emulsion is hereinafterreferred to as feed stock) were added respectively 2.2 wt % per PTFE ofthe emulsion stabilizers mentioned in Table 1. Under gentle agitation,water was added to obtain aqueous emulsions A, B and C containing about30 wt % PTFE.

                  TABLE 1                                                         ______________________________________                                        Sample    Kind of Emulsion Stabilizer                                         ______________________________________                                        A         C.sub.8 H.sub.17 --Ph--O--CH.sub.2 CH.sub.2 O--CH.sub.2                       CH.sub.2 SO.sub.3 Na                                                B             C.sub.8 H.sub.17 --Ph--O--(CH.sub.2 CH.sub.2 O).sub.2                     --CH.sub.2 CH.sub.2 SO.sub.3 Na                                     C             C.sub.9 H.sub.19 --Ph--O--(CH.sub.2 CH.sub.2 O).sub.9                     --CH.sub.2 CH.sub.2 OH                                              ______________________________________                                    

Mechanical stability test: The increased ratio of particle size measuredwith the aforementioned method for Samples A, B and C is shown in Table2. By comparison with Sample C (Comparative Example 1) using theconventional emulsion stabilizer p-nonylphenylpolylethyleneglycolether,Sample A (Example 1) and Sample B (Example 2) added with the emulsionstabilizer of the present invention showed better mechanical stability.

                  TABLE 2                                                         ______________________________________                                                        Average  Average                                                              Particle   Particle                                                                              Increased                                                  Size,         Size,                                                                                 Ratio of                                                before       after                                                                                  Particle                                                                        Mechanical                            Example Sample  Agitation                                                                               Agitation                                                                            Size    Stability                            ______________________________________                                        Example 1                                                                             A       0.261μ                                                                              0.265μ                                                                            1.5%   Good                                   Example 2                                                                                  B       0.261μ                                                                              0.263μ                                                                            0.8%                                                                                  Good                             Comparative                                                                            C           0.247μ                                                                              0.279μ                                                                            13.0%                                                                              Poor                                Example 1                                                                     ______________________________________                                    

EXAMPLE 3 AND COMPARATIVE EXAMPLES 2 and 3

Evaporating concentration test: Samples D, E and F were prepared byadding to the feed stock respectively 3.5 wt % per PTFE weight of theemulsion stabilizer shown in Table 3. The sample was subject toevaporating concentration under atmospheric pressure in a round bottomflask put on a 150 W mantle heater. Sample D (Example 3) wasconcentrated to obtain a 60 wt % PTFE aqueous emulsion though bubblingoccurred. Sample E (Comparative Example 2) was impossible to beconcentrated due to formation of a large amount of agllomerate. Sample F(Comparative Example 3) was impossible to be concentrated due tooccurrence of vigorous bubbling.

                  TABLE 3                                                         ______________________________________                                        Sample    Kind of Emulsion Stabilizer                                         ______________________________________                                        D         C.sub.8 H.sub.17 --Ph--O--(CH.sub.2 CH.sub.2 O).sub.2 --CH.sub.2               CH.sub.2 SO.sub.3 Na                                               E         C.sub.9 H.sub.19 --Ph--O--(CH.sub.2 CH.sub.2 O).sub.9 --CH.sub.2               CH.sub.2 OH                                                        F             C.sub.9 H.sub.19 --Ph--SO.sub.3 Na                              ______________________________________                                    

EXAMPLE 4 AND COMPARATIVE EXAMPLE 4

Thermal stability test: The thermal stability test was conducted againsta 60 wt % PTFE aqueous emulsion obtained in Example 3 as Sample D(Example 4) and a 60 wt % PTFE aqueous emulsion Sample G (ComparativeExample 4) prepared by use of 6.1 wt % per PTFE ofp-nonylphenylpolyethyleneglycolether (n=9) and concentration methodaccording to the U.S. Pat. No.3,037,953. The result obtained is shown inTable 4. The viscosity of Sample D increased gradually in accordancewith increase in temperature, but that of Sample G increased rapidlybetween 40° C. and 60° C.

                  TABLE 4                                                         ______________________________________                                                                              Ther-                                                                         mal                                     Example                                                                              Sam-   Viscosity cP            Stabi-                                  No.    ple    20° C.                                                                         30° C.                                                                       40° C.                                                                       50° C.                                                                       60° C.                                                                       lity                            ______________________________________                                        Example 4                                                                            D      21.5    25.5  25.5  31.5  34.0  Good                            Compa-    G        22.5                                                                               21.5                                                                                22.5                                                                                56.0                                                                                97.0                                                                                Poor                          tive                                                                          Example 4                                                                     ______________________________________                                    

EXAMPLE 5 AND COMPARATIVE EXAMPLE 5

Environmental pollution test: Sample B and Sample C each were dilutedwith pure water to adjust the concentration of emulsion stabilizer inthe emulsion to 100 ppm. Into a 200 ml beaker was placed 50 ml of thediluted Sample, 5 g of Ca(OH)₂ was added, agitated slowly for 1 hourwith a 4 cm magnetic stirrer, decanted and the concentration of emulsionstabilizer in the supernatant liquid was measured. The concentration ofemulsion stabilizer in the supernatant liquid of Sample B (Example 5)was 0, but that of Sample C (Comparative Example 5) was 1000 ppm.

INDUSTRIAL APPLICATION

An aqueous emulsion of fluorine-containing polymer according to thepresent invention has superior mechanical stability and thermalstability as well as decreased possibility of causing ground waterpollution in comparison with those emulsions using a conventionalemulsion stabilizer p-nonylphenylpolyethyleneglycolether.

What is claimed is:
 1. An aqueous emulsion comprising:a fluorine-containing polymer, which is a homopolymer of monomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and vinylidenefluoride, or a copolymer of said monomer, said aqueous emulsion being prepared by emulsion-polymerization of said monomer; and said emulsion further comprises an emulsion stabilizer represented by the formula:

    R--Ph--O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 --SO.sub.3 M

in which Ph is a phenyl radical, R is an alkyl radical having 8-12 carbon atoms, n is a number of 1-6, and M is Na, K or NH₄, wherein said stabilizer is added after the emulsion-polymerization of said monomer.
 2. The aqueous emulsion according to claim 1, in which said emulsion contains not less than 1 wt % of the emulsion stabilizer per the weight of polymer.
 3. An aqueous emulsion according to claim 1, in which the fluorine-containing polymer is polytetrafluoroethylene.
 4. An aqueous emulsion according to claim 1, in which the aqueous emulsion contains 1.5-5 wt % of the emulsion stabilizer per the weight of the polymer.
 5. An aqueous emulsion according to claim 1, in which the aqueous emulsion contains 1-75 wt % of the fluorine-containing polymer in colloidal particles having average particle size of 0.1-0.3μ. 